I don't know how often they're adding new titles to the deal, but to answer your second question, VMP will reach out to you with the deal offer about a month or so before your subscription ends. However, if you want to renew your sub and take the deal sooner than that, I think you just need to hit up customer service and they'll get you set up. As far as I know, the deal lasts until the end of this year so I guess if you let your sub lapse now but resubscribe before the end of December, you'll still qualify for the deal.
So for example my sub is up in August so I wouldn't expect to hear from VMP offering me the deal until probably around early July, unless I reach out to them first. And if I decide to not renew in August but jump back in with a multi-month sub in December, I'll still qualify for the deal.